Setting fuel pressure on Garmin G3X

If your engine is not running then MAP is equal to outside air pressure.  So in theory, the formula implies that your offset needs to equal outside air pressure, or 14.7 PSI at sea level on a standard pressure day. When you start the engine the MAP is reduced (especially at idle) and the offset is used in the formula to correct the gauge pressure to manifold referenced pressure.

 You can turn on your G3X to the calibration screen with the engine NOT running, then turn on a fuel pump and set the offset number so that the uncalibrated pressure is equal to the calibrated pressure.  This will be the correct setting for the current pressure altitude and conditions you are at - the number will probably be somewhere around 14.5.  Now start the engine and you will notice a significant difference in the uncalibrated reading and the calibrated reading, with the calibrated reading being most accurate. The pressure reading on the engine page should read correctly, and should be about the same as before the engine was started.

However, I have not been able to find out how (or if) the G3X compensates for altitude and atmospheric pressure changes.  It may incorporate altimeter information from its sensors, but the formula does not indicate that, nor does it remain accurate as altitude changes.  There may also be other pressure changes inside the engine compartment as flight conditions change which would affect the gauge pressure reading, thus requiring a slightly different offset to be fully accurate. When I used the procedure I described above at my airport, which is at 125’ above sea level, I would start getting a slight over pressure reading (48-49 PSI) about 9500’.  So what I ended up doing is calibrating the system at an airport with an altitude of 6000’, roughly 1/2 of my max flight altitude. I ended up with an offset of 12.5, which works well for my aircraft.  I have verified the pressure with a mechanical gauge installed at the fuel rail inlet and it is very close, both engine off and running.

I’m not an expert at this, so please don’t take this as a directive on how you should set up your aircraft.  I’m just sharing my experience in the hope that it may help get you headed in the right direction. Also, my sender is the Kavlico 50 PSI model, but I don’t think that should make a difference for this setting.  You do however need to select your sensor on your G3X.   

Jeff, Thank you for the detailed response!

I'll try your recommended setting on Sunday at the field and report the results.

Strange that there is no official documentation from Rotax on how to calibrate fuel pressure reference to manifold pressure...

Uri 

I don't see this as a Rotax issue because they don't recommend any specific fuel pressure sender or flight instrument package. If anything I think Garmin should address it more clearly, but in my opinion they really don't have this feature fully sorted out yet.  There are also fuel pressure senders on the market that reference manifold pressure directly so they avoid all this.  I would think those would be much more accurate. I have never used one, but I believe that type of sender unit has a small air tube that runs from the sensor to the air box for reference pressure.  

I sorted some of the mess I had.

I opened the cowl and checked for the fuel pressure sensor - it is a P4055 5025-3

Did some extensive search and found (only in Dynon catalog...) that it is same as P4055-50PSI G (not 75 PSI as was config)

The G stand for Gauge - which means it uses atmospheric pressure as its zero point (This makes perfect sense to Jeff's calibration instructions)

Performed the sequence described above:

1. turn on your G3X to the calibration screen with the engine NOT running

2. Set the fuel sensor to the correct 50 PSI + manifold as reference

3. turn on a fuel pump and set the offset number so that the uncalibrated pressure is equal to the calibrated pressure (sett offset to 1.0 bar)

4. start the engine 

Fuel pressure was right on the middle of the range of 3.0 bars

Jeff - thank you for the advise!!!

My only question now is, is a 50 PSI gauge safe? its very close to the work pressure

I know this is what Garmin is giving in their kit, but still...

One more note on the sensor

In the G3X installation manual, I see that the following sensors are mentioned for the 912 iS: